Silicon carbide single crystal substrate, silicon carbide epitaxial substrate, and method of manufacturing silicon carbide semiconductor device

ABSTRACT

A silicon carbide single crystal substrate includes a first main surface and an orientation flat. The orientation flat extends in a &lt;11-20&gt; direction. The first main surface includes an end region extending by at most 5 mm from an outer periphery of the first main surface. In a direction perpendicular to the first main surface, an amount of warpage of the end region continuous to the orientation flat is not greater than 3 μm.

TECHNICAL FIELD

The present disclosure relates to a silicon carbide single crystalsubstrate, a silicon carbide epitaxial substrate, and a method ofmanufacturing a silicon carbide semiconductor device. The presentapplication claims priority to Japanese Patent Application No.2015-228517 filed on Nov. 24, 2015, the entire contents of which areherein incorporated by reference.

BACKGROUND ART

Japanese Patent Laying-Open No. 2014-170891 (PTD 1) discloses a methodof epitaxially growing a silicon carbide layer on a silicon carbidesingle crystal substrate.

CITATION LIST Patent Document PTD 1: Japanese Patent Laying-Open No.2014-170891 SUMMARY OF INVENTION

A silicon carbide single crystal substrate according to the presentdisclosure includes a first main surface and an orientation flat. Theorientation flat extends in a <11-20> direction. The first main surfaceincludes an end region extending by at most 5 mm from an outer peripheryof the first main surface. In a direction perpendicular to the firstmain surface, an amount of warpage of the end region continuous to theorientation flat is not greater than 3 μm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view showing a construction of asilicon carbide single crystal substrate according to the presentembodiment.

FIG. 2 is a schematic plan view showing the construction of the siliconcarbide single crystal substrate according to the present embodiment.

FIG. 3 is a diagram showing a first example of relation between arelative height of a first main surface and a position on the first mainsurface.

FIG. 4 is an enlarged view of a region IV in FIG. 3.

FIG. 5 is a diagram showing a second example of relation between arelative height of the first main surface and a position on the firstmain surface.

FIG. 6 is a schematic diagram showing a construction of an apparatus formeasuring a relative height of the first main surface of the siliconcarbide single crystal substrate.

FIG. 7 is a schematic perspective view showing a construction of asilicon carbide epitaxial substrate according to the present embodiment.

FIG. 8 is a schematic plan view showing a construction of a firstmodification of the silicon carbide single crystal substrate accordingto the present embodiment.

FIG. 9 is a schematic plan view showing a construction of a secondmodification of the silicon carbide single crystal substrate accordingto the present embodiment.

FIG. 10 is a schematic plan view showing a construction of a thirdmodification of the silicon carbide single crystal substrate accordingto the present embodiment.

FIG. 11 is a schematic perspective view showing a first step of a methodof manufacturing a silicon carbide single crystal substrate according tothe present embodiment.

FIG. 12 is a schematic cross-sectional view showing a second step of themethod of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 13 is a schematic perspective view showing a third step of themethod of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 14 is a schematic cross-sectional view showing a fourth step of themethod of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 15 is a schematic perspective view showing a fifth step of themethod of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 16 is a schematic cross-sectional view showing a sixth step of themethod of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 17 is a schematic cross-sectional view showing a seventh step ofthe method of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment.

FIG. 18 is a schematic cross-sectional view showing a method ofmanufacturing a silicon carbide epitaxial substrate according to thepresent embodiment.

FIG. 19 is a flowchart schematically showing a method of manufacturing asilicon carbide semiconductor device according to the presentembodiment.

FIG. 20 is a schematic cross-sectional view showing a first step of themethod of manufacturing a silicon carbide semiconductor device accordingto the present embodiment.

FIG. 21 is a schematic cross-sectional view showing a second step of themethod of manufacturing a silicon carbide semiconductor device accordingto the present embodiment.

FIG. 22 is a schematic cross-sectional view showing a third step of themethod of manufacturing a silicon carbide semiconductor device accordingto the present embodiment.

DESCRIPTION OF EMBODIMENTS Overview of Embodiment of the PresentDisclosure

Overview of an embodiment of the present disclosure will initially bedescribed. In the description below, the same or corresponding elementshave the same reference characters allotted and the same descriptionthereof will not be repeated. Regarding crystallographic denotationherein, an individual orientation, a group orientation, an individualplane, and a group plane are shown in [ ], < >, ( ), and { },respectively. A crystallographically negative index is normallyexpressed by a number with a bar “-” thereabove, however, a negativesign herein precedes a number.

(1) A silicon carbide single crystal substrate 10 according to thepresent disclosure includes a first main surface 11 and an orientationflat 31. Orientation flat 31 extends in a <11-20> direction. First mainsurface 11 includes an end region 103 extending by at most 5 mm from anouter periphery 105 of first main surface 11. In a directionperpendicular to first main surface 11, an amount of warpage 101 of endregion 103 continuous to orientation flat 31 is not greater than 3 μm.

A silicon carbide single crystal substrate is normally obtained bycutting a silicon carbide single crystal ingot with a wire saw. Incutting a silicon carbide ingot with a wire saw, ideally, the wire sawis introduced substantially perpendicularly to a side surface of thesilicon carbide single crystal ingot. At the time of start of cutting,however, an angle of introduction of the wire saw with respect to theside surface of the silicon carbide single crystal ingot is not stableand the wire saw may obliquely be introduced into the side surface. Acarbon plane is physically cut more readily than a silicon plane.Therefore, the wire saw tends to move toward the carbon plane.Consequently, a cut surface of the silicon carbide single crystal ingot(in other words, a front surface and a rear surface of the cut siliconcarbide single crystal substrate) tends to be curved toward the carbonplane at the time of start of cutting of the silicon carbide singlecrystal ingot. Therefore, an outer edge of the front surface of the cutsilicon carbide single crystal substrate may be warped upward in adirection away from the rear surface of the silicon carbide singlecrystal substrate or downward in a direction toward the rear surface onthe contrary.

In particular, when an end region of the front surface of the siliconcarbide single crystal substrate continuous to an orientation flat isgreatly warped (specifically, more than 3 μm), a stacking fault tends todevelop from the orientation flat into a silicon carbide layer informing the silicon carbide layer on the silicon carbide single crystalsubstrate through epitaxial growth. Therefore, an amount of warpage ofthe end region (that is, an amount of warpage upward or downward) isdesirably reduced (specifically to 3 μm or smaller).

As a result of studies, the inventors have conceived of arranging aprotection portion in a form of a plate on an orientation flat of asilicon carbide single crystal ingot in cutting the silicon carbidesingle crystal ingot with a wire saw, thereafter cutting the protectionportion first, and in succession, cutting the silicon carbide singlecrystal ingot. Though a cut surface of the protection portion which iscut first may be curved with respect to a side surface of the protectionportion, the cut surface will gradually be substantially perpendicularto the side surface. Therefore, the silicon carbide ingot cut insuccession to the protection portion is cut substantiallyperpendicularly to the side surface thereof. Consequently, an amount ofwarpage of the end region can be reduced. Specifically, an amount ofwarpage of the end region can be not greater than 3 μm. Consequently, astacking fault which develops from the orientation flat into the siliconcarbide layer during epitaxial growth can be lessened.

(2) In silicon carbide single crystal substrate 10 according to (1),when a cross-section which divides orientation flat 31 perpendicularlyinto two equal sections when viewed in the direction perpendicular tofirst main surface 11 is viewed, toward orientation flat 31, end region103 may be warped upward in a direction away from a surface 13 oppositeto first main surface 11. Amount of warpage 101 may represent a distancebetween a point of contact 7 between orientation flat 31 and first mainsurface 11 and a point 6 where a least square line 4 calculated from across-sectional profile 15 of first main surface 11 in a region 102extending from a position distant by 3 mm from orientation flat 31toward a center 5 of first main surface 11 to a position distant by 5 mmtherefrom intersects with orientation flat 31.

(3) In silicon carbide single crystal substrate 10 according to (1),when a cross-section which divides orientation flat 31 perpendicularlyinto two equal sections when viewed in the direction perpendicular tofirst main surface 11 is viewed, toward orientation flat 31, end region103 may be warped downward in a direction toward surface 13 opposite tofirst main surface 11. Amount of warpage 101 may represent a distancebetween point of contact 7 between orientation flat 31 and first mainsurface 11 and point 6 where least square line 4 calculated fromcross-sectional profile 15 of first main surface 11 in region 102extending from a position distant by 3 mm from orientation flat 31toward center 5 of first main surface 11 to a position distant by 5 mmtherefrom intersects with a virtual plane 36 extending along orientationflat 31.

(4) In silicon carbide single crystal substrate 10 according to any of(1) to (3), amount of warpage 101 may be not greater than 2 μm.

(5) In silicon carbide single crystal substrate 10 according to (4),amount of warpage 101 may be not greater than 1 μm.

(6) A silicon carbide epitaxial substrate 100 according to the presentdisclosure may include silicon carbide single crystal substrate 10described in any of (1) to (5) and a silicon carbide layer 20. Siliconcarbide layer 20 is located on first main surface 11. Silicon carbidelayer 20 includes a second main surface 12 opposite to a surface 14 incontact with first main surface 11. Second main surface 12 is free froma stacking fault extending in a <1-100> direction from orientation flat31 and having a length not shorter than 1 mm.

(7) In silicon carbide single crystal substrate 10 according to any of(1) to (5), when a line segment 3 which divides orientation flat 31perpendicularly into two equal sections is divided into four equalsections when viewed in the direction perpendicular to first mainsurface 11, first main surface 11 may include a lower region 41extending from orientation flat 31 to a position 44 corresponding to ¼of line segment 3. Amount of warpage 101 of end region 103 continuous toan end portion 33 of lower region 41 is not greater than 3 μm.

(8) Silicon carbide epitaxial substrate 100 according to the presentdisclosure may include silicon carbide single crystal substrate 10described in (7) and silicon carbide layer 20. Silicon carbide layer 20is located on first main surface 11. Silicon carbide layer 20 includessecond main surface 12 opposite to surface 14 in contact with first mainsurface 11. Second main surface 12 is free from a stacking faultextending in a <1-100> direction from end portion 33 of lower region 41and having a length not shorter than 1 mm.

(9) In silicon carbide single crystal substrate 10 according to any of(1) to (5), when line segment 3 which divides orientation flat 31perpendicularly into two equal sections is divided into four equalsections when viewed in the direction perpendicular to first mainsurface 11, first main surface 11 may include an upper region 43extending from an end portion 35 opposite to orientation flat 31 to aposition 45 corresponding to ¼ of the line segment. Amount of warpage101 of end region 103 continuous to end portion 35 of upper region 43 isnot greater than 3 μm.

(10) Silicon carbide epitaxial substrate 100 according to the presentdisclosure may include silicon carbide single crystal substrate 10described in (9) and silicon carbide layer 20. Silicon carbide layer 20is located on first main surface 11. Silicon carbide layer 20 includessecond main surface 12 opposite to surface 14 in contact with first mainsurface 11. Second main surface 12 is free from a stacking faultextending in a <1-100> direction from end portion 35 of upper region 43and having a length not shorter than 1 mm.

(11) In silicon carbide single crystal substrate 10 according to any of(1) to (5), when line segment 3 which divides orientation flat 31perpendicularly into two equal sections is divided into four equalsections when viewed in the direction perpendicular to first mainsurface 11, first main surface 11 may include lower region 41 extendingfrom orientation flat 31 to position 44 corresponding to ¼ of the linesegment and upper region 43 extending from end portion 35 opposite toorientation flat 31 to position 45 corresponding to ¼ of the linesegment. Amount of warpage 101 of end region 103 continuous to endportion 33 of lower region 41 is not greater than 3 μm and amount ofwarpage 101 of end region 103 continuous to end portion 35 of upperregion 43 is not greater than 3 μm.

(12) Silicon carbide epitaxial substrate 100 according to the presentdisclosure may include silicon carbide single crystal substrate 10described in (11) and silicon carbide layer 20. Silicon carbide layer 20is located on first main surface 11. Silicon carbide layer 20 includessecond main surface 12 opposite to surface 14 in contact with first mainsurface 11. Second main surface 12 is free from a stacking faultextending in a <1-100> direction from end portion 33 of lower region 41and having a length not shorter than 1 mm and from a stacking faultextending in the <1-100> direction from end portion 35 of upper region43 and having a length not shorter than 1 mm.

(13) A method of manufacturing a silicon carbide semiconductor device300 according to the present disclosure includes steps below. Siliconcarbide epitaxial substrate 100 described in any of (6), (8), (10), and(12) is prepared. Silicon carbide epitaxial substrate 100 is processed.

DETAILS OF EMBODIMENT OF THE PRESENT DISCLOSURE

Details of an Embodiment of the Present Disclosure Will be DescribedBelow.

(Silicon Carbide Single Crystal Substrate)

A construction of a silicon carbide single crystal substrate accordingto the present embodiment will initially be described. As shown in FIGS.1 and 2, silicon carbide single crystal substrate 10 includes first mainsurface 11, a third main surface 13 opposite to first main surface 11,and a side end surface 30 located between first main surface 11 andthird main surface 13. Side end surface 30 is constituted of a planarorientation flat 31 and a curved curvature portion 32. Orientation flat31 extends in a <11-20> direction. Orientation flat 31 is substantiallyrectangular. A longitudinal direction of orientation flat 31 is the<11-20> direction. A direction perpendicular to orientation flat 31 maybe a <1-100> direction.

As shown in FIG. 2, when viewed in a direction perpendicular to firstmain surface 11, side end surface 30 includes linear orientation flat 31and curvature portion 32 in an arc shape. The center of a circumcircleof a triangle formed by any three points on curvature portion 32 may bedefined as center 5 of first main surface 11. Silicon carbide singlecrystal substrate 10 (which may be abbreviated as a “single crystalsubstrate” below) is made of silicon carbide single crystal. Polytype ofsilicon carbide single crystal is, for example, 4H—SiC. 4H—SiC is higherin other polytype in electron mobility and dielectric strength. Siliconcarbide single crystal substrate 10 contains an n-type impurity such asnitrogen (N). Silicon carbide single crystal substrate 10 has, forexample, an n conductivity type.

First main surface 11 is, for example, a {0001} plane or a surfaceinclined by at most 8° from the {0001} plane. When first main surface 11is inclined from the {0001} plane, a direction of inclination of anormal to first main surface 11 (an off direction) is, for example, the<11-20> direction. An angle of inclination (an off angle) from the{0001} plane may be not smaller than 1° or not smaller than 2°. The offangle may be not greater than 7° or not greater than 6°. First mainsurface 11 has a maximum diameter (a diameter), for example, not smallerthan 100 mm. The maximum diameter may be not smaller than 150 mm, notsmaller than 200 mm, or not smaller than 250 mm. The upper limit of themaximum diameter is not particularly limited. The upper limit of themaximum diameter may be set, for example, to 300 mm. When first mainsurface 11 is located on a side of a (0001) plane, third main surface 13is on a side of a (000-1) plane. In contrast, when first main surface 11is on the side of the (000-1) plane, third main surface 13 is on theside of the (0001) plane. First main surface 11 includes end region 103(a first end region) extending by at most 5 mm from outer periphery 105of first main surface 11 toward center 5 and a central region 104surrounded by end region 103. End region 103 is continuous toorientation flat 31.

An amount of warpage of the end region will now be described.

FIG. 3 shows cross-sectional profile 15 of first main surface 11 when across-section which divides orientation flat 31 perpendicularly into twoequal sections when viewed in the direction perpendicular to first mainsurface 11 is viewed. In FIG. 3, the ordinate represents a relativeheight of first main surface 11 and the abscissa represents a positionon the first main surface in a direction perpendicular to orientationflat 31. A method of measuring a relative height will be describedlater. A first position 1 refers to a position in the center of anorientation flat when viewed in the direction perpendicular to firstmain surface 11. A second position 2 refers to a position of curvatureportion 32 opposite to first position 1 when viewed from center 5 offirst main surface 11. As shown in FIG. 3, in a cross-sectional view,toward orientation flat 31, end region 103 may be warped upward in adirection away from third main surface 13 opposite to first main surface11. A relative height of first main surface 11 may be lowest at secondposition 2. A relative height of first main surface 11 may graduallyincrease from second position 2 toward first position 1.

FIG. 4 is an enlarged view of a region IV in FIG. 3. The abscissa inFIG. 4 represents a distance from orientation flat 31 in the directionperpendicular to orientation flat 31. A position 0 on the abscissarefers to the position of orientation flat 31. Least square line 4calculated from cross-sectional profile 15 of first main surface 11 inregion 102 extending from a position distant by 3 mm from orientationflat 31 toward center 5 of first main surface 11 to a position distantby 5 mm therefrom is assumed. Amount of warpage 101 represents adistance between point 6 where least square line 4 intersects withorientation flat 31 and point of contact 7 between orientation flat 31and first main surface 11. Contact point 7 may be a highest position inorientation flat 31 in the direction perpendicular to first main surface11.

As shown in FIG. 5, in a cross-sectional view, toward orientation flat31, end region 103 may be warped downward in a direction toward thirdmain surface 13 opposite to first main surface 11. The abscissa in FIG.5 represents a distance from orientation flat 31 in the directionperpendicular to orientation flat 31. Position 0 on the abscissa refersto the position of orientation flat 31. Least square line 4 calculatedfrom cross-sectional profile 15 of first main surface 11 in region 102extending from a position distant by 3 mm from orientation flat 31toward center 5 of first main surface 11 to a position distant by 5 mmtherefrom is assumed. Amount of warpage 101 represents a distancebetween a point 8 where least square line 4 intersects with virtualplane 36 extending along orientation flat 31 and point of contact 7between orientation flat 31 and first main surface 11. Contact point 7may be a lowest position in orientation flat 31 in the directionperpendicular to first main surface 11.

As set forth above, toward outer periphery 105, end region 103 may bewarped upward in the direction away from third main surface 13 ofsilicon carbide single crystal substrate 10 or may be warped downward inthe direction toward third main surface 13 on the contrary. Third mainsurface 13 includes a second end region extending by at most 5 mm froman outer periphery of third main surface 13. The second end region iscontinuous to orientation flat 31. When first end region 103 is warpedupward, the second end region may be warped downward. In contrast, whenfirst end region 103 is warped downward, the second end region may bewarped upward. In the direction perpendicular to first main surface 11,amount of warpage 101 of first end region 103 is not greater than 3Amount of warpage 101 is preferably not greater than 2 μm and morepreferably not greater than 1 μm. Similarly, in the directionperpendicular to third main surface 13, amount of warpage 101 of thesecond end region may be not greater than 3 μm, not greater than 2 μm,or not greater than 1 μm.

A method of measuring an amount of warpage will now be described.

An amount of warpage can be measured, for example, with a Dyvoce-seriessurface profiling system manufactured by Kohzu Precision Co., Ltd. Asshown in FIG. 6, a surface profiling system 57 mainly includes, forexample, a laser displacement meter 50 and an XY stage 55. Laserdisplacement meter 50 mainly includes a light emitting element 51 and alight receiving element 52. Light emitting element 51 is implemented,for example, by semiconductor laser.

As shown in FIG. 6, silicon carbide single crystal substrate 10 isarranged on XY stage 55. First main surface 11 of silicon carbide singlecrystal substrate 10 is irradiated with incident light 53 from lightemitting element 51. Reflected light 54 from first main surface 11 issensed by light receiving element 52. A distance from laser displacementmeter 50 to first main surface 11 can thus be measured. By moving XYstage 55 within a two-dimensional plane, a profile of a relative heightof a surface along a radial direction of first main surface 11 can bemeasured.

An angle θ between a straight line 58 perpendicular to first mainsurface 11 and a direction of incidence of incident light 53 is, forexample, not smaller than 0° and not greater than 60°. When the angle isgreater than 60°, noise is higher due to diffusion in particular aroundthe outer periphery of first main surface 11 and it is difficult toaccurately measure a surface profile of silicon carbide single crystalsubstrate 10. In the present embodiment, a surface profile around theouter periphery of first main surface 11 (in particular, a profilearound a tangent 7 between first main surface 11 and side end surface30) can accurately be measured by setting the angle to be smaller.

For example, a relative height of first main surface 11 is measuredalong line segment 3 in FIG. 2. Then, silicon carbide single crystalsubstrate 10 is shifted by 10 mm, for example, in the <11-20> directionwith the use of XY stage 55. Then, a relative height of first mainsurface 11 is measured along a line segment in parallel to line segment3. As set forth above, a profile of a relative height of first mainsurface 11 is measured at a 10-mm interval.

Amount of warpage 101 of end region 103 continuous to orientation flat31 being not greater than 3 μm means that amount of warpage 101 of endregion 103 at all measurement positions is not greater than 3 μm whenamount of warpage 101 of end region 103 is measured at measurementpositions at the 10-mm interval in a direction of extension oforientation flat 31 (that is, the <11-20> direction) when viewed in thedirection perpendicular to first main surface 11.

(Silicon Carbide Epitaxial Substrate)

A construction of a silicon carbide epitaxial substrate according to thepresent embodiment will now be described. As shown in FIG. 7, siliconcarbide epitaxial substrate 100 includes silicon carbide single crystalsubstrate 10 and silicon carbide layer 20. Silicon carbide layer 20 islocated on first main surface 11. Silicon carbide layer 20 includes afourth main surface 14 in contact with first main surface 11 and secondmain surface 12 opposite to fourth main surface 14. Second main surface12 is free from a stacking fault which extends in the <1-100> directionfrom orientation flat 31 and has a length not shorter than 1 mm. Thesecond main surface is preferably free from a stacking fault which has alength not shorter than 1.5 mm and more preferably free from a stackingfault which has a length not shorter than 2 mm. A length of a stackingfault is defined as a length in the <1-100> direction. Second mainsurface 12 may include a stacking fault which extends from a regiondifferent from orientation flat 31, a stacking fault which extends in adirection different from the <1-100> direction, or a stacking faulthaving a length shorter than 1 mm.

Silicon carbide layer 20 is an epitaxial layer formed through epitaxialgrowth. Silicon carbide layer 20 contains an n-type impurity such asnitrogen. A concentration of the n-type impurity contained in siliconcarbide layer 20 may be lower than a concentration of an n-type impuritycontained in silicon carbide single crystal substrate 10. Siliconcarbide layer 20 defines second main surface 12. Silicon carbide layer20 may have a thickness, for example, not smaller than 5 μm, not smallerthan 10 μm, or not smaller than 15 μm.

(Method of Observing Stacking Fault)

A method of observing a stacking fault will now be described. Forexample, a photoluminescence imaging apparatus manufactured by PhotonDesign Corporation is used for observation of a stacking fault. Whensecond main surface 12 of silicon carbide epitaxial substrate 100 isirradiated with excitation light, photoluminescence is observed insecond main surface 12. For example, white light is employed asexcitation light. White light passes, for example, through a 313-nm bandpass filter and is emitted to second main surface 12. Photoluminescencepasses, for example, through a 740-nm low pass filter and thereafterreaches a light receiving element such as a camera. As set forth above,a photoluminescence image of a measurement region on second main surface12 is shot.

For example, by shooting a photoluminescence image of second mainsurface 12 while silicon carbide epitaxial substrate 100 is moved in adirection in parallel to second main surface 12, the photoluminescenceimage over the entire second main surface 12 is mapped. White streakswhich linearly extend from orientation flat 31 are identified as astacking fault in the photoluminescence image.

(First Modification of Silicon Carbide Single Crystal Substrate)

As shown in FIG. 8, an example in which line segment 3 which dividesorientation flat 31 perpendicularly into two equal sections is dividedinto four equal sections in silicon carbide single crystal substrate 10when viewed in the direction perpendicular to first main surface 11 isassumed. First main surface 11 includes lower region 41 extending fromorientation flat 31 to position 44 corresponding to ¼ of line segment 3.Line segment 3 is located on first main surface 11. Line segment 3passes through center 5 of first main surface 11. Position 44 defines aline segment perpendicular to line segment 3. Position 44 is located ata position distant by a length corresponding to ¼ of line segment 3 fromfirst position 1 representing a point of contact between line segment 3and orientation flat 31 when line segment 3 is divided into four equalsections.

When viewed in the direction perpendicular to first main surface 11,curvature portion 32 in an arc shape includes a lower arc portion 33, acentral arc portion 34, and an upper arc portion 35. Central arc portion34 is located between lower arc portion 33 and upper arc portion 35.Lower arc portion 33 is defined by lower region 41. Amount of warpage101 of end region 103 continuous to end portion 33 of lower region 41 isnot greater than 3 μm. Amount of warpage 101 of end region 103 being notgreater than 3 μm means that amount of warpage 101 of end region 103 atall measurement positions is not greater than 3 μm when amount ofwarpage 101 of end region 103 is measured at measurement positions atthe 10-mm interval in a direction along end portion 33 when viewed inthe direction perpendicular to first main surface 11. Amount of warpage101 of end region 103 is preferably not greater than 2 μm and morepreferably not greater than 1 μm.

(First Modification of Silicon Carbide Epitaxial Substrate)

As shown in FIG. 7, silicon carbide epitaxial substrate 100 according toa first modification includes silicon carbide single crystal substrate10 according to the first modification and silicon carbide layer 20.Silicon carbide layer 20 is located on first main surface 11. Siliconcarbide layer 20 includes fourth main surface 14 in contact with firstmain surface 11 and second main surface 12 opposite to fourth mainsurface 14. Second main surface 12 is free from a stacking fault whichextends in the <1-100> direction from end portion 33 of lower region 41and has a length not shorter than 1 mm. Second main surface 12 mayinclude a stacking fault which extends from a region different from endportion 33 of lower region 41, a stacking fault which extends in adirection different from the <1-100> direction, or a stacking faulthaving a length shorter than 1 mm.

(Second Modification of Silicon Carbide Single Crystal Substrate)

As shown in FIG. 9, an example in which line segment 3 which dividesorientation flat 31 perpendicularly into two equal sections is dividedinto four equal sections in silicon carbide single crystal substrate 10according to a second modification when viewed in the directionperpendicular to first main surface 11 is assumed. First main surface 11includes upper region 43 extending from end portion 35 opposite toorientation flat 31 to position 45 corresponding to ¼ of the linesegment. Line segment 3 is located on first main surface 11. Linesegment 3 passes through center 5 of first main surface 11. Position 45defines a line segment perpendicular to line segment 3. Position 45 islocated at a position distant by a length corresponding to ¼ of linesegment 3 from second position 2 representing a point of contact betweenline segment 3 and end portion 35 when line segment 3 is divided intofour equal sections.

When viewed in the direction perpendicular to first main surface 11,curvature portion 32 in an arc shape includes lower arc portion 33,central arc portion 34, and upper arc portion 35. Central arc portion 34is located between lower arc portion 33 and upper arc portion 35. Upperarc portion 35 is defined by upper region 43. Amount of warpage 101 ofend region 103 continuous to end portion 35 of upper region 43 is notgreater than 3 μm. Amount of warpage 101 of end region 103 continuous toend portion 35 of upper region 43 being not greater than 3 μm means thatamount of warpage 101 of end region 103 at all measurement positions isnot greater than 3 μm when amount of warpage 101 of end region 103 ismeasured at measurement positions at the 10-mm interval in a directionalong end portion 35 when viewed in the direction perpendicular to firstmain surface 11. Amount of warpage 101 of end region 103 is preferablynot greater than 2 μm and more preferably not greater than 1 μm.

(Second Modification of Silicon Carbide Epitaxial Substrate)

As shown in FIG. 7, silicon carbide epitaxial substrate 100 according toa second modification includes silicon carbide single crystal substrate10 according to the second modification and silicon carbide layer 20.Silicon carbide layer 20 is located on first main surface 11. Siliconcarbide layer 20 includes second main surface 12 opposite to surface 14in contact with first main surface 11. Second main surface 12 is freefrom a stacking fault which extends in the <1-100> direction from endportion 35 of upper region 43 and has a length not shorter than 1 mm.Second main surface 12 may include a stacking fault which extends from aregion different from end portion 35 of upper region 43, a stackingfault which extends in a direction different from the <1-100> direction,or a stacking fault having a length shorter than 1 mm.

(Third Modification of Silicon Carbide Single Crystal Substrate)

As shown in FIG. 10, an example in which line segment 3 which dividesorientation flat 31 perpendicularly into two equal sections is dividedinto four equal sections in silicon carbide single crystal substrate 10according to a third modification when viewed in the directionperpendicular to first main surface 11 is assumed. First main surface 11includes lower region 41 extending from orientation flat 31 to position44 corresponding to ¼ of the line segment and upper region 43 extendingfrom end portion 35 opposite to orientation flat 31 to position 45corresponding to ¼ of the line segment. Line segment 3 is located onfirst main surface 11. Line segment 3 passes through center 5 of firstmain surface 11. Position 44 defines a line segment perpendicular toline segment 3. Position 44 is located at a position distant by a lengthcorresponding to ¼ of line segment 3 from first position 1 representingthe point of contact between line segment 3 and orientation flat 31 whenline segment 3 is divided into four equal sections. Position 45 islocated at a position distant by a length corresponding to ¼ of linesegment 3 from second position 2 representing the point of contactbetween line segment 3 and end portion 35 when line segment 3 is dividedinto four equal sections.

When viewed in the direction perpendicular to first main surface 11,curvature portion 32 in an arc shape includes lower arc portion 33,central arc portion 34, and upper arc portion 35. Central arc portion 34is located between lower arc portion 33 and upper arc portion 35. Upperarc portion 35 is defined by upper region 43. Lower arc portion 33 isdefined by lower region 41.

Amount of warpage 101 of end region 103 continuous to end portion 33 oflower region 41 is not greater than 3 μm. Amount of warpage 101 of endregion 103 continuous to end portion 33 of lower region 41 being notgreater than 3 μm means that amount of warpage 101 of end region 103 atall measurement positions is not greater than 3 μm when amount ofwarpage 101 of end region 103 is measured at measurement positions atthe 10-mm interval in a direction along end portion 33 when viewed inthe direction perpendicular to first main surface 11. Amount of warpage101 of end region 103 is preferably not greater than 2 μm and morepreferably not greater than 1 μm.

Amount of warpage 101 of end region 103 continuous to end portion 35 ofupper region 43 is not greater than 3 μm. Amount of warpage 101 of endregion 103 continuous to end portion 35 of upper region 43 being notgreater than 3 μm means that amount of warpage 101 of end region 103 atall measurement positions is not greater than 3 μm when amount ofwarpage 101 of end region 103 is measured at measurement positions atthe 10-mm interval in a direction along end portion 35 when viewed inthe direction perpendicular to first main surface 11. Amount of warpage101 of end region 103 is preferably not greater than 2 μm and morepreferably not greater than 1 μm.

(Third Modification of Silicon Carbide Epitaxial Substrate)

As shown in FIG. 7, silicon carbide epitaxial substrate 100 according toa third modification includes silicon carbide single crystal substrate10 according to the third modification and silicon carbide layer 20.Silicon carbide layer 20 is located on first main surface 11. Siliconcarbide layer 20 includes second main surface 12 opposite to surface 14in contact with first main surface 11. Second main surface 12 is freefrom a stacking fault which extends in the <1-100> direction from endportion 33 of lower region 41 and has a length not shorter than 1 mm andfree from a stacking fault which extends in the <1-100> direction fromend portion 35 of upper region 43 and has a length not shorter than 1mm. Second main surface 12 may include a stacking fault which extendsfrom a region different from end portion 35 of upper region 43 and endportion 33 of lower region 41, a stacking fault which extends in adirection different from the <1-100> direction, or a stacking faulthaving a length shorter than 1 mm.

(Method of Manufacturing Silicon Carbide Single Crystal Substrate)

A method of manufacturing a silicon carbide single crystal substrateaccording to the present embodiment will now be described.

A silicon carbide single crystal ingot 80 of polytype 4H ismanufactured, for example, with a sublimation method. As shown in FIG.11, silicon carbide single crystal ingot 80 includes an upper surface81, a lower surface 82, and a side surface 83. Side surface 83 islocated between upper surface 81 and lower surface 82. Side surface 83is provided continuously to upper surface 81 and provided continuouslyto lower surface 82. Upper surface 81 is, for example, convexly curved.Lower surface 82 is, for example, substantially planar and substantiallyannular. In a cross-sectional view, side surface 83 has a widthincreasing from lower surface 82 toward upper surface 81. When siliconcarbide single crystal is manufactured with the sublimation method withthe use of a crucible, upper surface 81 faces a silicon carbide sourcematerial and lower surface 82 faces a seed substrate.

Silicon carbide single crystal ingot 80 is then shaped. Specifically, afirst grindstone 61 and a second grindstone 62 are prepared. Firstgrindstone 61 is arranged to face side surface 83. Second grindstone 62is arranged to face upper surface 81. Lower surface 82 of siliconcarbide single crystal ingot 80 is fixed to a holder 65. Holder 65 ismade, for example, of stainless steel. By rotating holder 65 around anaxis of rotation 67, silicon carbide single crystal ingot 80 fixed toholder 65 is rotated. As first grindstone 61 is pressed against sidesurface 83 of silicon carbide single crystal ingot 80 while siliconcarbide single crystal ingot 80 is rotated, side surface 83 is ground.Similarly, as second grindstone 62 is pressed against upper surface 81of silicon carbide single crystal ingot 80 while silicon carbide singlecrystal ingot 80 is rotated, upper surface 81 is ground. Silicon carbidesingle crystal ingot 80 is thus formed into a substantially columnarshape (see FIG. 13). Silicon carbide single crystal ingot 80 hassubstantially annular upper surface 81, substantially annular lowersurface 82, and substantially cylindrical side surface 83. Holder 65 isremoved from lower surface 82 of silicon carbide single crystal ingot80.

Then, a holder 71 is attached to lower surface 82 of silicon carbidesingle crystal ingot 80. While silicon carbide single crystal ingot 80is fixed to holder 71 at rest, a third grindstone 68 is pressed againstside surface 83 of silicon carbide single crystal ingot 80. As thirdgrindstone 68 is pressed against in a direction 70 shown with an arrowwhile it carries out reciprocating motion along a direction 69 inparallel to side surface 83, side surface 83 is ground. An orientationflat 84 is thus formed in silicon carbide single crystal ingot 80 (seeFIG. 15). Side surface 83 is defined by planar orientation flat 84 and acurved surface portion 85. Orientation flat 84 is substantiallyrectangular.

Then, a protection portion 92 is attached to orientation flat 84.Protection portion 92 is provided to cover the entire planar orientationflat 84 (see FIG. 16). Protection portion 92 is fixed to orientationflat 84, for example, with an adhesive. Though a shape of protectionportion 92 is not particularly limited, it is, for example, in a form ofa plate. For example, silicon carbide is employed as a material forprotection portion 92. Protection portion 92 may be composed of singlecrystal silicon carbide or polycrystalline silicon carbide. Protectionportion 92 in a form of a plate has a thickness, for example, notsmaller than 10 mm.

As shown in FIG. 16, silicon carbide single crystal ingot 80 is held ona base 91 while orientation flat 84 is covered with protection portion92. A surface 96 of base 91 is provided with a recess 95 in an arcshape. Curved surface portion 85 of side surface 83 of silicon carbidesingle crystal ingot 80 is arranged in recess 95. Silicon carbide singlecrystal ingot 80 is fixed to base 91, for example, with an adhesive.Orientation flat 84 of silicon carbide single crystal ingot 80 isinclined with respect to surface 96 of base 91. Protection portion 92may be distant from surface 96.

Silicon carbide single crystal ingot 80 is then cut. As shown in FIG.17, a wire saw 93 is arranged on a side opposite to base 91 when viewedfrom silicon carbide single crystal ingot 80. As base 91 moves in adirection 94 shown with an arrow while wire saw 93 swings, siliconcarbide single crystal ingot 80 is cut with wire saw 93. A plurality ofwire saws 93 may be aligned in a direction perpendicular to uppersurface 81. Wire saw 93 comes in contact with protection portion 92before it comes in contact with silicon carbide single crystal ingot 80.After a part of protection portion 92 is cut, wire saw 93 starts to cutsilicon carbide single crystal ingot 80.

In cutting silicon carbide single crystal ingot 80 with wire saw 93,ideally, wire saw 93 is substantially perpendicularly introduced intoside surface 83 of silicon carbide single crystal ingot 80. At the timeof start of cutting, however, an angle of introduction of wire saw 93into side surface 83 of silicon carbide single crystal ingot 80 is notstable and wire saw 93 may obliquely be introduced into side surface 83.When cutting of silicon carbide single crystal ingot 80 with wire saw 93is started without using protection portion 92, wire saw 93 tends to beintroduced obliquely to side surface 83.

In the present embodiment, protection portion 92 covers the entiresurface of orientation flat 84. Though a cut surface of protectionportion 92 which is cut first may be curved with respect to the sidesurface of protection portion 92, the cut surface will gradually besubstantially perpendicular to the side surface. Therefore, siliconcarbide single crystal ingot 80 cut in succession to protection portion92 is cut substantially perpendicularly to side surface 83 of siliconcarbide single crystal ingot 80. Consequently, amount of warpage 101 ofend region 103 continuous to orientation flat 31 can be reduced (seeFIG. 4). Silicon carbide single crystal substrate 10 (see FIG. 1) isprepared as above.

(Method of Manufacturing Silicon Carbide Epitaxial Substrate)

A method of manufacturing a silicon carbide epitaxial substrate will nowbe described. Silicon carbide layer 20 is formed on silicon carbidesingle crystal substrate 10 through epitaxial growth, for example,through hot-wall chemical vapor deposition (CVD). Specifically, siliconcarbide single crystal substrate 10 is arranged in a CVD reactionchamber. For example, after a pressure in the reaction chamber islowered from the atmospheric pressure to approximately 1×10⁻⁶ Pa,increase in temperature of silicon carbide single crystal substrate 10is started. During temperature increase, hydrogen (H₂) gas representingcarrier gas is introduced into the reaction chamber.

After a temperature in the reaction chamber reaches, for example,approximately 1600° C., source gas and doping gas are introduced intothe reaction chamber. The source gas includes Si source gas and C sourcegas. For example, silane (SiH₄) gas can be used as the Si source gas.For example, propane (C₃H₈) gas can be used as the C source gas. Flowrates of the silane gas and the propane gas are set, for example, to 46sccm and 14 sccm, respectively. A volume ratio of the silane gas tohydrogen is set, for example, to 0.04%. A C/Si ratio of the source gasis set, for example, to 0.9.

For example, ammonia (NH₃) gas is used as the doping gas. The ammoniagas is thermally decomposed more easily than nitrogen gas having triplebond. By using the ammonia gas, improvement in in-plane uniformity of aconcentration of carriers can be expected. A concentration of theammonia gas with respect to the hydrogen gas is set, for example, to 1ppm. By introducing the carrier gas, the source gas, and the doping gasinto the reaction chamber while silicon carbide single crystal substrate10 is heated to approximately 1600° C., silicon carbide layer 20 isformed on silicon carbide single crystal substrate 10 through epitaxialgrowth (see FIGS. 7 and 18). Silicon carbide epitaxial substrate 100including silicon carbide single crystal substrate 10 and siliconcarbide layer 20 is thus manufactured.

(Method of Manufacturing Silicon Carbide Semiconductor Device)

A method of manufacturing silicon carbide semiconductor device 300according to the present embodiment will now be described.

The method of manufacturing a silicon carbide semiconductor deviceaccording to the present embodiment mainly includes an epitaxialsubstrate preparation step (S10: FIG. 19) and a substrate processingstep (S20: FIG. 19).

Initially, the epitaxial substrate preparation step (S10: FIG. 19) isperformed. Specifically, with the method of manufacturing a siliconcarbide epitaxial substrate described previously, silicon carbideepitaxial substrate 100 is prepared (see FIGS. 7 and 18).

Then, the substrate processing step (S20: FIG. 19) is performed.Specifically, a silicon carbide semiconductor device is manufactured byprocessing the silicon carbide epitaxial substrate. “Processing”includes various types of processing such as ion implantation, heattreatment, etching, formation of an oxide film, formation of anelectrode, and dicing. The substrate processing step may include atleast any processing of ion implantation, heat treatment, etching,formation of an oxide film, formation of an electrode, and dicing.

A method of manufacturing a metal oxide semiconductor field effecttransistor (MOSFET) representing one example of a silicon carbidesemiconductor device will be described below. The substrate processingstep (S20: FIG. 19) includes an ion implantation step (S21: FIG. 19), anoxide film forming step (S22: FIG. 19), an electrode forming step (S23:FIG. 19), and a dicing step (S24: FIG. 19).

Initially, the ion implantation step (S21: FIG. 19) is performed. Ap-type impurity such as aluminum (Al) is implanted into second mainsurface 12 where a mask (not shown) provided with an opening is formed.A body region 132 having the p conductivity type is thus formed. Then,an n-type impurity such as phosphorus (P) is implanted into a prescribedposition in body region 132. A source region 133 having the nconductivity type is thus formed. Then, a p-type impurity such asaluminum is implanted into a prescribed position in source region 133. Acontact region 134 having the p conductivity type is thus formed (seeFIG. 20).

A portion in silicon carbide layer 20 other than body region 132, sourceregion 133, and contact region 134 is a drift region 131. Source region133 is spaced away from drift region 131 by body region 132. Ions may beimplanted by heating silicon carbide epitaxial substrate 100 to atemperature approximately not lower than 300° C. and not higher than600° C. After ion implantation, silicon carbide epitaxial substrate 100is subjected to activation annealing. The impurities implanted insilicon carbide layer 20 are activated through activation annealing sothat carriers are generated in each region. An atmosphere for activationannealing may be, for example, an argon (Ar) atmosphere. A temperaturefor activation annealing may be set, for example, to approximately 1800°C. A time period for activation annealing may be set, for example, toapproximately 30 minutes.

Then, the oxide film forming step (S22: FIG. 19) is performed. Forexample, as silicon carbide epitaxial substrate 100 is heated in anatmosphere containing oxygen, an oxide film 136 is formed on second mainsurface 12 (see FIG. 21). Oxide film 136 is composed, for example, ofsilicon dioxide (SiO₂). Oxide film 136 functions as a gate insulatingfilm. A temperature for thermal oxidation treatment may be set, forexample, to approximately 1300° C. A time period for thermal oxidationtreatment may be set, for example, to approximately 30 minutes.

After oxide film 136 is formed, heat treatment may further be performedin a nitrogen atmosphere. For example, heat treatment may be performedapproximately for one hour at approximately 1100° C. in a nitric oxide(NO) or nitrous oxide (N₂O) atmosphere. Further thereafter, heattreatment may be performed in the argon atmosphere. For example, heattreatment may be performed in the argon atmosphere for approximately onehour at a temperature approximately from 1100 to 1500° C.

Then, the electrode forming step (S23: FIG. 19) is performed. A firstelectrode 141 is formed on oxide film 136. First electrode 141 functionsas a gate electrode. First electrode 141 is formed, for example, withCVD. First electrode 141 is composed, for example, of polysilicon whichis conductive by containing an impurity. First electrode 141 is formedat a position where it faces source region 133 and body region 132.

Then, an interlayer insulating film 137 which covers first electrode 141is formed. Interlayer insulating film 137 is formed, for example, withCVD. Interlayer insulating film 137 is composed, for example, of silicondioxide. Interlayer insulating film 137 is formed as being in contactwith first electrode 141 and oxide film 136. Then, oxide film 136 andinterlayer insulating film 137 at a prescribed position are etched away.Source region 133 and contact region 134 are thus exposed through oxidefilm 136.

A second electrode 142 is formed in that exposed portion, for example,with sputtering. Second electrode 142 functions as a source electrode.Second electrode 142 is composed, for example, of titanium, aluminum,and silicon. After second electrode 142 is formed, second electrode 142and silicon carbide epitaxial substrate 100 are heated at a temperature,for example, approximately from 900 to 1100° C. Second electrode 142 andsilicon carbide epitaxial substrate 100 are thus brought in ohmiccontact with each other. Then, an interconnection layer 138 is formed asbeing in contact with second electrode 142. Interconnection layer 138 iscomposed of a material containing, for example, aluminum.

Then, a third electrode 143 is formed on third main surface 13. Thirdelectrode 143 functions as a drain electrode. Third electrode 143 iscomposed, for example, of an alloy containing nickel and silicon (forexample, NiSi).

Then, the dicing step (S24: FIG. 19) is performed. For example, assilicon carbide epitaxial substrate 100 is diced along a dicing line,silicon carbide epitaxial substrate 100 is divided into a plurality ofsemiconductor chips. Silicon carbide semiconductor device 300 ismanufactured as above (see FIG. 22).

Though the method of manufacturing a silicon carbide semiconductordevice according to the present disclosure is described above withreference to a MOSFET, the manufacturing method according to the presentdisclosure is not limited as such. The manufacturing method according tothe present disclosure is applicable to various silicon carbidesemiconductor devices such as an insulated gate bipolar transistor(IGBT), a Schottky barrier diode (SBD), a thyristor, a gate turn offthyristor (GTO), and a PiN diode.

It should be understood that the embodiment disclosed herein isillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than theembodiment above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

-   -   1 first position; 2 second position; 3 line segment; 4 least        square line; 5 center; 6, 8 point; 7, 9 contact point, tangent;        10 single crystal substrate; 11 first main surface; 12 second        main surface; 13 third main surface (surface); 14 fourth main        surface (surface); 15 cross-sectional profile; 20 silicon        carbide layer; 30 side end surface; 31, 84 orientation flat; 32        curvature portion; 33 lower arc portion (end portion); 34        central arc portion; 35 upper arc portion (end portion); 36        virtual surface; 41 lower region; 43 upper region; 50 laser        displacement meter; 51 light emitting element; 52 light        receiving element; 53 incident light; 54 reflected light; 55        stage; 57 surface profiling system; 61 first grindstone; 62        second grindstone; 65, 71 holder; 67 axis of rotation; 68 third        grindstone; 80 single crystal ingot; 81 upper surface; 82 lower        surface; 83 side surface; 85 curved surface portion; 91 base; 92        protection portion; 93 wire saw; 95 recess; 96 surface; 100        silicon carbide epitaxial substrate; 101 amount of warpage; 102        region; 103 end region (first end region); 104 central town        area; 105 outer periphery; 131 drift region; 132 body region;        133 source region; 134 contact region; 136 oxide film; 137        interlayer insulating film; 138 interconnection layer; 141 first        electrode; 142 second electrode; 143 third electrode; and 300        silicon carbide semiconductor device

1. A silicon carbide single crystal substrate comprising: a first mainsurface; and an orientation flat extending in a <11-20> direction, thefirst main surface including an end region extending by at most 5 mmfrom an outer periphery of the first main surface, and in a directionperpendicular to the first main surface, an amount of warpage of the endregion continuous to the orientation flat being not greater than 3 μm.2. The silicon carbide single crystal substrate according to claim 1,wherein when a cross-section which divides the orientation flatperpendicularly into two equal sections when viewed in the directionperpendicular to the first main surface is viewed, toward theorientation flat, the end region is warped upward in a direction awayfrom a surface opposite to the first main surface, and the amount ofwarpage represents a distance between a point of contact between theorientation flat and the first main surface and a point where a leastsquare line calculated from a cross-sectional profile of the first mainsurface in a region extending from a position distant by 3 mm from theorientation flat toward a center of the first main surface to a positiondistant by 5 mm from the orientation flat intersects with theorientation flat.
 3. The silicon carbide single crystal substrateaccording to claim 1, wherein when a cross-section which divides theorientation flat perpendicularly into two equal sections when viewed inthe direction perpendicular to the first main surface is viewed, towardthe orientation flat, the end region is warped downward in a directiontoward a surface opposite to the first main surface, and the amount ofwarpage represents a distance between a point of contact between theorientation flat and the first main surface and a point where a leastsquare line calculated from a cross-sectional profile of the first mainsurface in a region extending from a position distant by 3 mm from theorientation flat toward a center of the first main surface to a positiondistant by 5 mm from the orientation flat intersects with a virtualplane extending along the orientation flat.
 4. The silicon carbidesingle crystal substrate according to claim 1, wherein the amount ofwarpage is not greater than 2 μm.
 5. The silicon carbide single crystalsubstrate according to claim 4, wherein the amount of warpage is notgreater than 1 μm.
 6. A silicon carbide epitaxial substrate comprising:the silicon carbide single crystal substrate according to claim 1; and asilicon carbide layer on the first main surface, the silicon carbidelayer including a second main surface opposite to a surface in contactwith the first main surface, and the second main surface being free froma stacking fault extending in a <1-100> direction from the orientationflat and having a length not shorter than 1 mm.
 7. The silicon carbidesingle crystal substrate according to claim 1, wherein when a linesegment which divides the orientation flat perpendicularly into twoequal sections is divided into four equal sections when viewed in thedirection perpendicular to the first main surface, the first mainsurface includes a lower region extending from the orientation flat to aposition corresponding to ¼ of the line segment, and the amount ofwarpage of the end region continuous to an end portion of the lowerregion is not greater than 3 μm.
 8. A silicon carbide epitaxialsubstrate comprising: the silicon carbide single crystal substrateaccording to claim 7; and a silicon carbide layer on the first mainsurface, the silicon carbide layer including a second main surfaceopposite to a surface in contact with the first main surface, and thesecond main surface being free from a stacking fault extending in a<1-100> direction from the end portion of the lower region and having alength not shorter than 1 mm.
 9. The silicon carbide single crystalsubstrate according to claim 1, wherein when a line segment whichdivides the orientation flat perpendicularly into two equal sections isdivided into four equal sections when viewed in the directionperpendicular to the first main surface, the first main surface includesan upper region extending from an end portion opposite to theorientation flat to a position corresponding to ¼ of the line segment,and the amount of warpage of the end region continuous to the endportion of the upper region is not greater than 3 μm.
 10. A siliconcarbide epitaxial substrate comprising: the silicon carbide singlecrystal substrate according to claim 9; and a silicon carbide layer onthe first main surface, the silicon carbide layer including a secondmain surface opposite to a surface in contact with the first mainsurface, and the second main surface being free from a stacking faultextending in a <1-100> direction from the end portion of the upperregion and having a length not shorter than 1 mm.
 11. The siliconcarbide single crystal substrate according to claim 1, wherein when aline segment which divides the orientation flat perpendicularly into twoequal sections is divided into four equal sections when viewed in thedirection perpendicular to the first main surface, the first mainsurface includes a lower region extending from the orientation flat to aposition corresponding to ¼ of the line segment and an upper regionextending from an end portion opposite to the orientation flat to aposition corresponding to ¼ of the line segment, and the amount ofwarpage of the end region continuous to an end portion of the lowerregion is not greater than 3 μm and the amount of warpage of the endregion continuous to the end portion of the upper region is not greaterthan 3 μm.
 12. A silicon carbide epitaxial substrate comprising: thesilicon carbide single crystal substrate according to claim 11; and asilicon carbide layer on the first main surface, the silicon carbidelayer including a second main surface opposite to a surface in contactwith the first main surface, and the second main surface being free froma stacking fault extending in a <1-100> direction from the end portionof the lower region and having a length not shorter than 1 mm and from astacking fault extending in the <1-100> direction from the end portionof the upper region and having a length not shorter than 1 mm.
 13. Amethod of manufacturing a silicon carbide semiconductor devicecomprising: preparing the silicon carbide epitaxial substrate accordingto claim 6; and processing the silicon carbide epitaxial substrate.